In the design of circuitry, a signal is usually transmitted from one circuit layer to another through a via which is electrically coupled to the two circuit layers. The conventional coaxial via comprises a signal conductor, a dielectric member surrounding the periphery of the signal conductor, and a circular ground conductor surrounding the periphery of the dielectric member.
However, the space of layout of circuitry is limited on a substrate, so the dimension of the coaxial via is relatively smaller, and therefore the impedance of the signal conductor of the coaxial via is smaller than the impedance of a transmission wire (The impedance of a general transmission wire is 50 ohms). The different impedance between the transmission wire and the signal conductor of the coaxial via causes the reflection effect of signal transmission when signal is transmitted between the circuit layer and the signal conductor of the coaxial hole, especially when the signal transmits at high speed, the signal distortion occurs seriously. Therefore, it is important to reduce the reflection effect of the signal transmission in order to enhance the quality of the signal transmission.